Histidase from rat liver has been purified. The kinetic properties of the enzyme will be studied. An effort will be made to identify an inhibitor of histidase that can be administered in vivo in order to study the effect on histidine metabolism of blocking this major pathway for its degradation. Branched-chain keto acid concentrations in blood and tissues will be measured in an effort to determine whether, upon administration of a load of leucine to an animal, the keto acid formed in peripheral tissues is transported to liver for oxidation.